1. Field of the Invention
The present invention relates, in general, to metallization schemes for microcircuit interconnections of electronic devices and, more particularly, to PtSi/TiW/TiW(N)/Au (Type I) and PtSi/TiW/TiW(N)/TiW/Au (Type II) gold interconnections of semiconductor devices and associated processes of metallization.
2. Discussion of Related Art
In general, high performance and a higher degree of integration of semiconductor devices require reduction of overall dimensions, including the dimensions of metal leaders and contact hole diameters (including via diameters). This reduction in dimensions causes increases in chip operating temperatures and current densities in the metal stripes and interfaces between silicon and the metal conductors in the contact holes.
At the same time, a decrease in contact hole diameter consequently increases its aspect ratio (i.e., step height/diameter), and hence worsens the step coverage of the metals. A smaller step coverage in the contact holes causes further increases of the current density in the contact holes.
High power transistors, in particular, generate large currents and develop heat. The high current density and high temperature operations of the devices result in rather fast degradation of an aluminum alloy (commonly including copper and silicon) based interconnections due to an electromigration effect. The electromigration, caused by momentum transfer of electrons, causes formation of voids and an interruption in the metal conductors.
Gold is a good conductor and is more than seven times larger in atomic weight than aluminum. Gold based interconnections are very resistant to the electromigration effect. However, because of the Au--Si eutectic (.about.370.degree. C.), diffusion barriers are needed which should possess some essential properties, such as:
long-term high temperature stability,
good electromigration resistance with a high current density, good adhesion to both insulating films and gold conductor,
a good conductor,
low contact resistance to doped (n+ and p+) silicon, good step coverage in contact holes, non-particulation, and
a wide process latitude even for small dimensions (.about.1 .mu.m).
Gold based interconnections can usually be formed in two ways. First, a lift-off technique based on evaporation of diffusion barriers and gold can be used, e.g., a lift-off of Ti/Pt/Au metals. Second, a sputtering technique combined with plasma etched gold or electroplated gold conductors can be used.
One of the major advantages of the lift-off technique is that it rather easy to form metal pitches below 2 .mu.m. A successful lift-off technique requires an electron-beam evaporator with a large distance between metal sources and wafers. For instance, for 6" diameter wafers the distance must be larger than 145 cm in order to obtain an incident angle deviation of the evaporated metals less than 3 degrees from the normal to the wafer surface. Consequently, the size of the evaporator will be huge and the waste of precious metals will be very large. Furthermore, a requirement on the photoresist profile is quite high: a thick (&gt;2.5 .mu.m) and reentrant profile having negatively sloped sidewalls (&gt;90.degree.) as shown in FIG 1c. The technique yields very low step coverage and consistently proves very sensitive to particles with respect to the reliability of the diffusion barriers.
The sputtering and electroplating techniques combined with plasma etching is easily feasible with common equipment and have no difficulty with a large wafer diameter (&gt;6") in contrast to a lift-off technique. However, a drawback with this technique is the difficulty in shrinking a metal-pitch below 4 .mu.m due to some serious process barriers, such as:
photoresist loss in an electroplating bath,
geometry dependence of plating speed and difficulty in plating fine lines together with large size geometry,
too rough Au-conductor surfaces after a wet-etch of the sputtered Au-film used for electroplating and protection of diffusion barriers, and
etch undercuts at the diffusion barriers.
Even by a plasma etch technique it is difficult to eliminate some undercuts because the plated gold layer must act as an etch mask instead of a photoresist.
Gold is much softer than diffusion barriers, and hence it is difficult to apply a strong physical sputtering effect in a plasma etch process. At the same time the gold etch mask can not offer any side passivation with organic polymers like a photoresist etch mask. Therefore it is very important to suppress the etch undercut for the microcircuit interconnections.
In the past titanium-tungsten or/and nitrided titanium-tungsten, herein denoted as TiW and TiW(N) (which do not stand for a stoichiometric composition but for a "pseudo-alloy" of Ti.sub.x W.sub.1-x obtained from a cathode composition of X=0.30 (corresponding to Ti=10 wt %)), have been utilized. The actual composition of the sputtered TiW- and TiW(N)-films varies by sputtering parameters, and accordingly varies the film properties as a diffusion barrier and adhesion layers, etc.
Previously, the diffusion barriers of TiW and TiW(N) have been used in the metallization systems of Si/TiW/Al or PtSi/TiW/Al as IC-interconnections or Al(bondpad)/TiW(N)--TiW/Au (wire or bump) for packages.
J. A. Cunningham et al., "Corrosion Resistance of Several Integrated Circuit Metallization Systems," IEEE Transactions on Reliability, Vol. R-19, No. 4, November 1970, pp.182-187, discusses a metallization system of Al(bondpad)/TiW/Au, and R. S. Nowicki et al., "Studies of the TiW/Au metallization on Aluminum," Thin Solid Films, 53 (1978), pp. 195-205, discusses a metallization system of Al(bondpad)/TiW(N)/Au. The Nowicki et al. article includes observations about extensive intermixing of TiW and Au at 300.degree. C. for 6 minutes when the TiW-film was relatively pure. They also observed that a reactively sputtered film of TiW(N,O) improved the diffusion barrier property by orders of magnitude. Nitrogen and oxygen contents in the TiW(N,O)-film were .about.42 and .about.8 at. % (atomic percent), respectively. The oxygen atoms in the TiW(N,O)-film were unintentionally involved from residual gases in the sputter chamber. The TiW(N,O)-film even had minimal intrinsic stress, otherwise a TiW(O)-film with O .about.4 at. % had tensile stress.
Two patents to R. K. Sharma et al. disclose the use of a double-layer diffusion barrier of TiW(N)/TiW in a metallization system of Al(bondpad)/TiW(N)(1.5-3.0 k.ANG.)/TiW(0.5-1.0 k.ANG.)/Au(2-7 k.ANG.) for TAB (tape automated bonding) or wire bonding applications. See U.S. Pat. Nos. 4,880,708 and 4,927,505 dated Nov. 14, 1989 and May 22, 1990, respectively. Preferable thickness intervals are given in the parentheses. The TiW(N) film was obtained by sputtering with a gas mixture consisting of at least 30 vol % N.sub.2 in Ar. There is no information about the sputter technique.
The major reason to include Ti into W is to improve the adhesion property to SiO.sub.2 -layer because strong Ti--O bonds could be expected at the interface. The maximum solubility of Ti into W is only about 10 at. % at 600.degree. C. Any excess Ti will be microscopically distributed in a "pseudo-alloy" of TiW, and even located at grain boundaries and surfaces. The substantial improvement of the diffusion barrier property could be achieved by oxidation or/and nitridation of the Ti-atoms during sputtering of TiW-cathode (target). The formation of stable TiO.sub.2 and TiN at grain boundary particularly seems to effectively slow down important grain boundary diffusion. However, one of major drawbacks of the oxidation and nitridation of the Ti-atoms is the loss or reduction in the ability of forming Ti--O bonds at the interface between SiO.sub.2 and TiW.
For the package applications, Cunningham et al. and Nowicki et al. have been successful in using TiW(N) directly on top of Al-bondpads because of good adhesion between Al and W which can form intermetallic alloys with each other. Furthermore, one could easily apply strong sputter etch on the Al-surface in order to eliminate Al.sub.2 O.sub.3 and on the passivation film. The strong sputter etch could roughen the surfaces, and hence could enhance adhesion. In any way, some lateral dimension losses of a few micrometers in an etch due to weak adhesion between TiW(N) and the passivation film of SiO.sub.2 (PSG, BPSG glasses) or Si.sub.3 N.sub.4 can not influence reliability because usually the diffusion barrier covers nearly 10 .mu.m over the passivation film.
However, the situation is quite different in the microcircuit interconnections where a metal-pitch can be as small as 1.5 .mu.m. The adhesion strength between a SiO.sub.2 -layer with contact holes (&lt;1 .mu.m) and a diffusion barrier has a decisive influence on the size of the etch undercut and reliability. In this case the coverage of the diffusion barrier over the contact hole edge (FIG. 1e) may be a few tenths of a micron where one-tenth micron is very important. The application of a sputter etch is very limited due to very shallow depths of junctions for HF (high frequency) power transistors and high speed devices in general.
Dening et al., "Reliability of High Temperature I.sup.2 L Integrated Circuits," IEEE/International Reliability Physics Symposium Proc., 1984, pp.30-36, discusses a metallization system of PtSi/TiW(250 .ANG.)/TiW(N)(2 k.ANG.)/TiW(250 .ANG.)/Au(5 k.ANG.)/TiW(500 .ANG.) for IC-interconnection. The article mentions a lift-off technique applied to 3" wafers. There are no details about the lift-off technique in the paper. However, there are some serious feasibility problems in IC-production.
According to the present inventor's experience as well as that reported in the Dening et al. article, the adhesion strength between SiO.sub.2 and TiW(N) was not sufficiently strong to withstand the stress caused by thermal mismatching because the deposition temperature of TiW(N) may be in a range of 100.degree.-350.degree. C. and a difference of TCE (thermal coefficient of expansion) can be 4-8 ppm/.degree.C. This weak adhesion caused unacceptably large etch undercuts and oftentimes left a small cavity at a sidewall of the contact holes. The formation of the cavity seriously caused reliability problem because Au-diffusion into the bulk-Si as well as Si-diffusion into the Au-layer was observed after annealing at 420.degree. C. for 30 min. The mechanism seems to be surface diffusion.
K. A. Lorenzen et al., in U.S. Pat. No. 5,173,449 dated Dec. 22, 1992 entitled "Metallization Process", disclose a metallization process for a scheme of TiW(0.2-0.8 k.ANG.)/TiW(N)(2-5 k.ANG.)/TiW(0.2-0.8 k.ANG.)/Au(5-20 k.ANG.)/TiW(1-4 k.ANG., as an etch mask for Au-patterning and being etched away later) for applications of microdevice interconnections. Major process steps are based on sputtering and plasma etching techniques. However, the process latitude (feasibility) for patterning the fine line Au-conductors (1-1.25 .mu.m width or less) seems to be low because the plasma etching of a thick Au-layer is based on pure physical sputtering by argon and oxygen gases. Consequently, an HNO.sub.3 -boil is recommended after the Au-etch in order to clean the etch residues of Au, Ti and W-metals. However, the nitric acid boil can easily attack the TiW-layer unless the TiW-surface had already been oxidized.
The oxidation of the TiW-surface at the same time causes an oxidation of the etch residues of Ti- and W-metals from the eroded etch mask. Consequently, the cleaning procedure is ineffective and risky. The oxidation step can potentially cause an adhesion failure between the Au- and TiW-layers, especially with fine line Au-conductors.
Lorenzen et al. in U.S. Pat. No. 5,173,449 describe in detail the parameter settings of a sputter system (MRC-603, Material Research Corp., N.Y.), especially about N.sub.2 -gas purge procedure after a reactive sputtering of the TiW(N)-film. However, there is no description about the applied sputter technique which is of decisive importance for the reactive sputtering of the TiW(N)-film. They use a N.sub.2 /Ar-gas mixture of which the N.sub.2 -concentration is in the range of 14-38 vol %. The metallization scheme proposed by the Lorenzen et al. patent is not optimum with respect to contact resistance and high temperature stability because bottom layer of TiW has direct contact with the Si substrate.